Gamma ray (GR) detectors have been widely used to collect information during geological exploration, hydrocarbon drilling operations, etc. For example, GR detectors are commonly used to passively collect gamma ray radiation in the environment in which the GR detectors are disposed. Various geological media, such as shale, provides a natural source of gamma ray energy, whereas other geological media, such as sand, provides very little gamma ray energy. Accordingly, a GR detector is useful in obtaining information regarding the geological media and structure.
GR tools employing the aforementioned GR detectors have been commonly employed to create well logs useful in analyzing geological structures penetrated by hydrocarbon drilling operations. Such GR tools have comprised wireline tool configurations, which require removal of the drill string from the borehole in order to introduce the tool into the well. More recently, such GR tools have comprised logging while drilling (LWD) tool configurations, wherein the GR tool is included in the drill string and provides a GR detector disposed upon the circumference of the drill string assembly (i.e., eccentric from the drill string center of rotation). The foregoing GR tools comprise a passive GR detector, such as may be comprised of a scintillation detector, providing frequency and amplitude information corresponding to the gamma rays impinging thereon. Accordingly, by collecting gamma ray information, using the aforementioned GR tools, information regarding the geological structure, such as depth, thickness, and type of sediment beds, may be obtained. That is, gamma ray signatures associated with various media may be analyzed to determine the characteristics of a geological formation being explored.
Although GR logs acquired using the foregoing GR tools can reveal sedimentary structure of formations penetrated by the borehole, the information provided by such GR tools is not robust. Accordingly, where a vertical well penetrates horizontal structure, the GR logs may be relatively straight forward to interpret, such as by determining the measured depths (MDs) at which particular gamma ray signatures are experienced. However, where there is a high dip angle between the borehole and structure feature, such as sediment bed boundary, as is often experienced in high angle and horizontal (HA/HZ) wells, the information provided by GR logs becomes very difficult to analyze. Moreover, the eccentricity associated with the GR detector being disposed on the circumference of the drill stem further obscures the proper analysis of GR log information provided by LWD tools. For example, it has been discovered that a single sediment bed boundary penetrated by a borehole at a high dip angle (e.g., 80°) using a LWD GR tool provides a double peaked GR detector amplitude response. Such a response, without more information appears to represent more than one sediment bed boundary, none of which appear to be at the actual measured depth of the actual sediment bed boundary penetrated. As HA/HZ wells, and other situations where high dip angles are experienced, are becoming more common, the usefulness of GR logs is decreasing.
Efforts have been made to provide modeling or simulation of GR tool response in order to better interpret GR logs. For example, computer code providing Monte-Carlo for N particles (MCNP) simulation, developed by Los Alamos National Laboratory, has been used to simulate GR tool response. Unfortunately, such MCNP simulation requires substantial computing power and time. For example, simulation of GR tool response for a relatively simple formation often requires days of computing time on a multi-processor supercomputer. Moreover, a nuclear physicist, or other person with very advanced training, is required to properly implement the MCNP simulation. Accordingly, such simulation has been cost and time prohibitive for widespread use.